Contribution to the kinetic study of the dehydration of sorbitol over acidic ion-exchange resins

Abstract

Currently, the use of petroleum derivatives as a base in the commodity and fuel industry poses environmental and economic disadvantages of real concern. Therefore, the search for a substitute that can satisfy the current demands for this form of energy in a sustainable and cost-effective way has become a focus of global attention and research. Biomass, a green source of organic and plant-based resources, has proven to be a very promising candidate. Specifically, isosorbide is one of the molecules from biomass that has been gaining interest over the years. Nowadays, it is used in industrial processes in the pharmaceutical and cosmetics sectors, but its most important application is in the manufacture of polymers due to its appreciable rigidity in its structure. The most commonly used process to obtain this compound consists of a double dehydration of sorbitol to 1,4-sorbitan, the intermediate product, and from this to isosorbide. Sorbitol is a polyol that is quite common in nature and with a high market value. The reaction is carried out using an acid catalyst, currently sulphuric acid, which generates several drawbacks, such as corrosion of the devices used and a large expenditure of energy. Therefore, there is a desire to find solid catalysts able to overcome all these obstacles while providing good performance values and efficient isosorbide production. Several solid catalysts have already been investigated, e.g. ion exchange resins have been shown to be viable catalysts in this process, especially CT-482 offering high yield values at 190ºC. In this work, an analysis of the conditions under which the effects of catalyst loading, and internal and external mass transfers can be considered negligible is carried out by means of a set of preliminary experiments. Under such conditions, the kinetics of sorbitol dehydration was studied at different initial reactant concentrations and temperatures. As a result, activation energies of 90 ± 10 kJ/mol were obtained for the consumption of sorbitol and 100 ± 18 kJ/mol for the formation of isosorbide, the latter being the rate-controlling step of the reaction. In addition, an empirical modelling of the rate of isosorbide formation as a function of temperature and initial amount of reactant was performed. It was observed that the influence of the former is clearly much more dominant than the latter. However, a trend was shown that if one dependent variable increased, the effect of the other on the response variable was significantly higher.